Whether you sweat for an hour a week or an hour a day, there are a lot of factors that affect how much you’re really going to gain from gym. One of the most important: Consistency and timing. You have to stick with a routine to see results—but not for too long or you won’t be challenging your muscles enough.

“Initially you need consistency to see where you stand and to track your progress, but ultimately your muscles need confusion in order to continue to grow,” says Josey Greenwell, trainer at Barry’s Bootcamp in New York City. The longer you do a single move, the more your muscles become used to it. Over time, that adaptation means you’ll stop seeing gains in the mirror.

Most experts subscribe to the strategy of changing your workout routine every six to eight weeks to avoid this plateau, but you can’t rely just on the calendar. “Your body and your mind will tell you when it’s time to change your routine around and challenge your muscles more,” says LA-based personal trainer Lalo Fuentes, C.S.C.S. Check out these six red flags.

When you start a new routine, you feel the burn in the first set and by the third, you’re probably struggling to finish. The longer you do the routine, the easier this becomes—until it’s too easy. “During the exercises, you need to feel challenged in order to see results,” Fuentes says. If you’re shooting for 8 to 12 reps per set, but suddenly you can do 15 on the second and third set, it means your muscles have adapted too much and it’s time to increase your weight or change your routine, he explains.

Another way to gauge: “The last 3 reps should be the hardest reps to complete,” says Greenwell. If this isn’t the case (and all the reps kinda feel the same), it’s time to switch it up.

“On the psychological side, sheer boredom is a cue that it’s time to change it up,” says Justin Sanchez, trainer at YG Studios in New York. A big part of exercise adherence is maintaining that enjoyment factor when exercising, he explains. When you lose that, you’re going to lose the motivation to push through the last few sets, compromising your strength gains, and are at risk of losing the motivation to stick to your routine in the bigger picture.

Don’t have that drive to demolish everything on the dinner table? “After a good workout, your body is repairing the muscle fiber that gets broken down during the workout. It needs protein and nutrients to generate new muscle protein strands—hence the reason why we feel hungry when we have a good workout,” says Fuentes. “If your workouts are easy, then there are no muscle fibers to repair and our bodies don’t ask for extra food.”

One of the easiest ways to tell if your body has adapted to a routine is your heart rate. “If you are monitoring your heart rate and it’s staying relatively low when it should be working very hard, most likely you have adapted to your routine,” says Sanchez. To ensure your body is being challenged, aim to keep your HR at 60-80% of your max for most of any HIIT or cardio routine. Once it drops below this for the majority of a workout, drop the tired routine.

“Every time you hit the weights you should feel a small level of soreness,” says Fuentes. When you start a new routine or get back into the gym, your next-day aches are going to be pretty painful or the first week or two. This will dissipate the more your muscles adapt to the moves and stress. “Even the fittest people will be a bit sore after a great workout when they haven’t yet adapted to it,” Sanchez says. Once that ache in your abs or soreness in your quads is gone, that’s good indicator that you need to increase the weight, repetitions, or overhaul the routine entirely, Fuentes adds.

The mirror doesn’t lie: You should see the physical effects of a workout four to six weeks after hitting it consistently. Nutrition plays a huge part in this, Sanchez says, but if you are keeping your diet on track but are still not seeing any physical changes, the most likely culprit is adaptation.